Projection exposure tools for microlithography are used to produce micro-structured components using a photolithographic method. Here a structure-carrying mask—the so-called reticle—is illuminated with the aid of an illumination system and imaged onto a photosensitive layer with the aid of projection optics. The illumination system comprises a light source which makes available radiation with an appropriate wavelength, and illumination optics comprising different components which serve to make available uniform illumination with a predetermined angular distribution at the location of the structure-carrying mask. The structure-carrying mask illuminated in this way is imaged onto a photosensitive layer with the aid of the projection optics.
Here the minimum structure width that can be imaged with the aid of this type of projection optics is determined, among other things by the wavelength of the imaging radiation used. The smaller the wavelength of the imaging radiation, the smaller the structures that can be imaged with the aid of the projection optics. Nowadays imaging radiation with the wavelength of 193 nm or imaging radiation with a wavelength in the extreme ultraviolet range (EUV) are used. When using imaging light with a wavelength of 193 nm both refractive optical elements and reflective optical elements are used within the projection exposure tool. In contrast, when using imaging light in the EUV wavelength range only reflective optical elements (mirrors) are used. With a projection exposure tool for microlithography it is necessary for irradiation conditions to remain unchanged on the photosensitive layer during the whole operation. Only in this way can microstructured components of uniform quality be produced. Therefore, the irradiation conditions on the structure-carrying mask must also be as invariable as possible.
However, during operation various influences can change the irradiation conditions on the structure-carrying mask and the photosensitive layer. This can e.g. be heating of the reflective optical elements which thereupon slightly change their position or shape. Furthermore, it is possible for the radiation source to change during continuous operation, i.e. for the position of the light source to shift slightly. Furthermore, e.g. contaminations can also lead to the reflectivity of individual or of all of the mirrors changing. All of these influences lead to a change in the irradiation conditions on the structure-carrying mask and on the photosensitive layer. For this reason it is necessary to constantly monitor the irradiation conditions during operation. Here the measuring device provided for the monitoring should be designed such that monitoring is made possible without having to halt operation of the projection exposure tool for this purpose.
In United States patent application publication 2008/0151221 A1 a reflective optical element is proposed for this purpose with which the reflective coating is partially interrupted in order to guide the radiation striking these points onto a measuring device. The disadvantage of this, however, is that the reflective optical element now has regions which are no longer reflective. Furthermore, only the portion of radiation which is not reflected and so does not contribute to the irradiation conditions on the structure-carrying mask and the photosensitive layer, is monitored.